It's hard for me to give you any sort of definitive list since I'm a member of the press, and once celebrities know this about me they automatically go into "my publicist trained me to be this way around you" mode. But I have met a number of celebs outside the usual press junket atmosphere, and till this day the nicest, most personable celebrity I've ever come across was Jesse Eisenberg (Zombieland, Adventureland, Holy Rollers) ... so keep that in mind in case you ever run into him; he's good people. Other cool, down-to-earth personalities (that I've come across) include Robert Downey Jr. (and especially his wife, Susan), Kevin Smith, Jess Weixler, Joseph Gordon-Levitt, Zoe Lister-Jones, Matt Reeves, Malin Akerman, Emmanuelle Chriqui, Chuck Palahniuk, Morgan Spurlock and Seann William Scott. My opinion, in case you're curious, is based on me being able to have an actual human conversation with these people.
I could go on naming names, but I'd rather open this one up to you folks. Who were the nicest celebrities you've ever met in person?